FIG. 1A is a circuit diagram of a prior art differential programmable gain amplifier 100 with input 101. The differential gain amplifier 100 includes a multiplying digital to analog converter (DAC) 102, first current-to-voltage converter 107, second current-to-voltage converter 108, first differential amplifier 109 and second differential amplifier 110. The input signal of differential programmable gain amplifier 100 is received by input 101. Current-to-voltage converters 107 and 108 translate the portion of the input current remaining after passing through DAC 102 into a voltage signal. The voltage signal from the outputs of current-to-voltage converters 107 and 108 are applied to inputs of the first differential amplifier 109 and the second differential amplifier 110. First differential amplifier 109 and the second differential amplifier 110 sense the voltage between outputs of current-to-voltage converters 107 and 108 and produce differential output on outputs 115 and 120. However, this configuration requires an undesirably high number of components and high tolerances. In order for differential gain amplifier 100 to operate efficiently, components of differential gain amplifier 100 must be matched. By way of example, differential gain amplifier 100 requires matching of components, such as current-to-voltage converters 107 and 108. In addition, differential gain amplifier 100 requires four operational amplifiers to produce the differential output. There is a desire for a differential amplifier circuit that does not require high tolerances of the components in differential amplifiers nor so many amplifying circuits in the signal path. There is also a desire for a differential amplifier that maintains a high common-mode rejection ratio, low control signal interference and low signal distortion.
FIG. 1B shows an exemplary output spectrum 150 for differential programmable gain amplifier 100 with a symmetrical drive. Differential programmable gain amplifier 100 compensates distortion caused by nonlinearity of MOSFET switches of DAC 102. Resistance of a closed MOSFET switch is a function of applied voltage. Full distortion compensation takes place when the signals on inputs have equal amplitude and opposite phase. By way of example, an input signal, such as a pure 1 kHz sine wave, having signals on input 101 with equal amplitude and opposite phase will generate spectrum 150 having with peak 155.
FIG. 1C shows an exemplary output spectrum 160 for differential programmable gain amplifier 100 with asymmetrical drive. For example, an input signal having a pure 1 kHz sine wave, with the input signal applied to a single input terminal or input 102 and second input signal terminal shorted to ground, will generate output spectrum 160 having peak 165 and second order distortion shown by peak 166. Differential programmable gain amplifier 100 does compensated second order distortion.